Current aircraft have three levels of piloting equipment:                a first level of equipment consisting of flight controls acting directly on the control surfaces and the engines,        a second level of equipment consisting of the automatic pilot and/or the flight director acting on the flight controls, directly for the automatic pilot or via the pilot for the flight director, to slave the aircraft to a flight parameter such as, for example, heading, roll, pitch, altitude, speed, etc., and        a third level of equipment consisting of the flight management computer (FMS) which is capable of generating a flight plan and acting on the automatic pilot or the flight director to have it followed by the aircraft.        
The flight management computer FMS, hereinafter called flight computer FMS, has, among other main functions, the generation and the automatic following of a flight plan, a flight plan consisting of lateral and vertical trajectories that the aircraft must follow to go from the position that it occupies to its destination, and the speeds with which these trajectories must be travelled.
The generation of a flight plan is done, among other things, from imposed waypoints associated with altitude, time and speed constraints. These imposed waypoints and their associated constraints are introduced, into the flight computer FMS, by an operator of the aircraft, for example a member of the crew of the aircraft, by means of cockpit equipment with keyboard and screen providing the man-machine interface, such as that known by the acronym MCDU, standing for Multipurpose Control and Display Unit. The generation of the flight plan proper, consists in constructing the lateral and vertical trajectories of the flight plan from a sequence of straight line segments starting from a departure point, passing through imposed waypoints and culminating at an arrival point, observing standardized construction rules and taking into account altitude and speed constraints associated with each imposed waypoint.
A flight plan is made up of terminal procedures and successive “route” segments, also called “airways”. The terminal procedures are located in the vicinity of the airports: they define the aircraft landing and take off procedures. The routes form a network linking “nodes” linked by meshes. The aircraft generally follow the meshes of this network.
FIG. 1 represents a route map or “airway map”: A route is defined by means of an identifier consisting of a succession of alphanumeric characters such as, for example, “UL856”, “UL608”.
FIG. 2 represents an exemplary flight plan linking the Paris airport to the Melbourne airport. The flight plan takes the form of a table consisting of rows that we identify by a row index i. These rows define successive segments SGi forming the flight plan.
In the example shown in FIG. 2, the table comprises around 30 rows defining as many flight plan segments. Each row of the table has three columns: a right hand column, a central column and a left hand column.
The right hand column of row i of the table thus contains an identifier of an exit point PAAi from the segment SGi.
The left hand column of row i of the table contains an indicator of the type of segment to which the exit point PAAi belongs (for example AWY for “Airway”, DIR for “DIRECT”).
Finally, the central column of row i of the table optionally contains a route identifier to be followed to reach the exit point PAAi (for example the name of the SID, of the airway, etc.).
The routes are stored in the database BDR onboard the aircraft. These are straight lines defined by a list of auxiliary route points.
When the exit point PAAi is a geographic reference point which is not part of the auxiliary route points stored in the database BDR, the join indicator is set to “DIR”, and the central column is empty. The terms “direct trajectories” or “DIRECT” segment apply.
When the exit point is a geographic point which is part of the auxiliary route points stored in the BDR, the join indicator is set to “AWY”. In this case, the central column of row i contains the identifier of the route chosen to link the exit point of row i−1 and the exit point of row i.
In particular situations such as an aircraft take off or landing, the join indicator is set to “SID” or “APP”.
A flight plan assigned to the aircraft is read and is built row by row. Thus, the flight plan represented in FIG. 2 begins with a take off procedure “SID” with which to join with the point PILUL along the route PIL1HL.
The aircraft must then be directed successively by direct trajectories to two geographic reference points (“TINIL” and “DERAK”), then from “DERAK”, the aircraft follows the route “UL856” to the junction point between the routes “UL856” and “UL608” named “KPT”. From the junction point “KPT”, it then bifurcates on the route “UL608”.
Then, the aircraft exits from the route “UL608” at “ERKIR” then performs a DIRECT to “GOLVA” where it enters the following route (“UL 604”), and so on, until the position of the arrival airport is reached.
Routes are entered into an FMS of recent design through an alphanumeric keyboard, by an operator manually entering the identifier of the routes.
FIG. 3a illustrates an example of “routes” pages of an FMS which is presented to an aircraft operator on an MCDU. This page enables an operator to enter into the FMS the information needed to generate a flight plan.
The flight plan generation methods according to the prior art are based on successive determinations of segments forming the flight plan. In the example represented in FIG. 3a, the operator has chosen, from the point “FISTO”, the route “UN874”, then the route “UT191” that is, he has used an alphanumeric keyboard to successively enter these two identifiers.
From the first two routes entered, the flight plan generation method according to the prior art automatically determines a junction point “BAMES”.
Subsequently, an input, or a selection of the route identifier “UT426”, enables the flight plan generation method to determine the junction point “ABUDA”. The operator finally chooses an exit point from the last route: “DIMAL”.
In the most recent FMS systems which include a “windows” type interface, commonly called MFD (Multi Function Display), the entry of a route to generate a flight plan is based on a route selection by means of a drop-down menu offering the operator a list of route identifiers, as represented in FIG. 3b. 
For example, the drop-down menu presented to the operator on an MFD presents to the operator a limited number, for example ten, route identifiers in alphabetical order of the identifiers. This presentation in particular enables the operator to choose from the displayed identifiers that which corresponds to the route that suits him to build a segment of his flight plan.
In certain situations, for example, to react to an unfavourable change of weather conditions encountered in flight, an aircraft operator, for example a pilot, may have to very rapidly generate a new flight plan. In these situations, a route selection made via a display of a limited number of route identifiers in alphabetical order is unsuitable because it can be relatively lengthy.
In particular, a flight plan, as has been seen, can comprise several tens of segments which require as many manual route selections as are sequentially required. A manual selection can take several minutes. Furthermore, the manipulations required by the operator to select a route are lengthy and tedious when the number of routes passing through a node exceeds the display capability of the drop-down menu.
Finally, in a stress situation, it is difficult for the operator to choose a relevant route simply by studying the route identifiers. This type of presentation does not help lighten the workload of the operator. Finally, the limited number of characters (5) generates major input error potential (for example AWY UL508 and UL608, and so on), given the high number of airways in the navigation databases).